Anyway to compare function aliases? Or any ideas?

[Marco de Wild]

On Thursday, 4 July 2019 at 15:10:05 UTC, aliak wrote:
> Any ideas on how to be able to do something like this?
>
> struct S(alias _fun) {
>   alias Fun = _fun;
> }
>
> void algorithm(alias f, T)(T s) {
>   static if (&f == &T.Fun) {
>     // trivial return
>   } else {
>     // must perform work, then return
>   }
> }
>
> Can you use function addresses in some way? I've seen that some 
> of them are resolved to __lambda0 and other times to a function 
> type, i.e.
>
> Error: incompatible types for (& f) is (& __lambda1): void 
> function(A!(function () => 3) t) and int function() pure 
> nothrow @nogc @safe
>
> That comes from doing doing this:
>
> alias g = () => 3;
> algorithm!g(S!g());
>
> I've thought of something along the lines of a function alias 
> wrapper. But I'm not sure how to make that work either. 
> Something like:
>
> struct Fun(alias _fun, string _id) {
>   alias Fun = _fun;
>   enum ID = _id;
> }
>
> Then maybe something like:
>
> alias g = () => 3;
> Fun!(g, "myid") gfun;
> algorithm!gfun(S!gfun());
>
> Then inside algorithm the check becomes:
>
> static if (&f.ID == &T.Fun.ID)
>
> But then I'd like to generate the ID at instantiation point. So 
> is using __LINE__, __FILE__ and applying some hash function a 
> good idea here?
>
> Any other ideas?
>
> Cheers,
> - Ali
>
> PS: If you're curious of a semi working sample, to see what I'm 
> actually trying to do, I've put up this gist that does not 
> compile right now:
>
> https://gist.github.com/aliak00/fcdd4fa7512035405bb7015cf6d8016f

I don't know if it will solve your whole problem, but have you 
tried __traits(isSame, W0.fun, fun)?

Reduced example:

struct Foo(alias fun){
     alias bar = fun;
}

void stuff(alias fun, T)(T t)
{
     static if(__traits(isSame, fun, T.bar)) {
         pragma(msg, "Yes");
     } else {
      	pragma(msg, "No");
     }
}

void a(){}
void b(){}

void main()
{
     stuff!a(Foo!a()); // Yes
     stuff!a(Foo!b()); // No
}